1. Technical Field
The present invention relates in general to systems for soldering and in particular to a method and apparatus for depositing solder at multiple sites on a circuit board. Still more particularly, the present invention relates to a method and apparatus for automated solder deposition at multiple sites wherein a uniform amount and thickness of solder may be deposited at each of a plurality of conductive pads.
2. Description of the Related Art
Solder distribution onto mounting pads for surface mount boards has generally been accomplished in the semiconductor industry utilizing a screening process. In a screen process artwork and screens must be fabricated having the solder deposition pattern. Then, a precise alignment process is carried out wherein the solder is screened onto the surface mount pads. The solder paste used for this process requires a substantially long cure and bake time. Thus, in addition to the necessary complexity of the alignment process, this prior art technique is relatively time consuming.
The prior art solder distribution technique utilizing screening is further complicated by a requirement that the pattern mix very fine lead pitch and with surface mount pads along with standard surface mount pads. For example, in the case of tape automated bonding, the pitches vary from four to twenty mils, while standard surface mount parts have pitches in the range of twenty to fifty mils. Thus, this process requires the different amounts of solder be distributed on various parts of the board. Given the precision required, it is common to utilize separate screening steps, one for very fine lead pitch and with surface mount parts and a second for standard surface mount parts. There is also a possibility of damaging the solder deposited in a previous step when multiple screening operations are carried out. Additionally, screening fine line solder presents a problem because the solder paste tends to stick in the openings of the screen, as the openings get progressively narrower.
The prior art also discloses other techniques for depositing solder across the surface of a printed circuit. Dip soldering and wave soldering are both techniques which are known in the prior art. Wave soldering involves pumping a molten solder through a nozzle to form a standing wave. In this process, the entire side of an assembly containing printed conductors with the leads from the circuit components projecting through various points generally travels at a predetermined rate of speed over the standing surface of the wave of molten solder. The lower surface of the assembly is placed into contact with the upper fluid surface of the wave.
By this technique, the solder wave in the first instance wets the joining surfaces and promotes through hole penetration. This in turn helps to assure the formation of reliable solder joints and fillets. Wave soldering is illustrated in U.S. Pat. Nos. 3,705,457 and 4,360,144. An example of an immersion technique is illustrated in U.S. Pat. No. 4,608,941 wherein panels are immersed in a liquid solder bath and then conveyed to an air knife which levels the molten solder on the panels. The air knife is therefore used to effectively clear the panels of excess solder and only the printed patterns retain the solder.
Another example of a solder leveler is contained in U.S. Pat. No. 4,619,841. The technique disclosed therein is used in conjunction with dip soldering techniques. Other techniques of selective deposition of solder onto printed circuit patterns are described in U.S. Pat. Nos. 4,206,254, 4,389,771 and 4,493,856.
U.S. Pat. No. 3,661,638 is also directed to a system for leveling and controlling the thickness of a conductive material on the walls of through-holes of a printed circuit board. That technique for removing the excess amount of conductive material employs heating to melt a conductive material after it has been deposited. Then, while the conductive material is in a plastic state, gyrating the board to cause the plastic material to move circumferentially about the through-hole and flow axially through the through-hole. More recently, several techniques have been proposed which utilizes a solder nozzle which deposits solder onto solder wettable contact pads in substantially uniform amounts on each pad. The tool utilized with such a nozzle generally comprises a solder reservoir or plenum, a heating element to melt the solder, and at the bottom of the reservoir, a foot which contains the nozzle and which passes over the contact pads to be wetted with solder. Examples of these types of systems are disclosed in U.S. Pat. No. 4,898,117, filed Apr. 15, 1988, entitled "Solder Deposition System" and U.S. patent application Ser. No. 07,586,655, now U.S. Pat. No. 5,042,708, filed of even date herewith entitled "Solder Placement Nozzle Assembly."
One problem which exists with the solder nozzle techniques described above is attributable to the manner in which these solder nozzles are controlled and manipulated by means of a robotic arm. Contact pads for the mounting of integrated circuit devices are typically disposed on a circuit board in linear arrays, typically in parallel rows or in a square or rectangle. During manipulation of solder nozzle utilizing a robotic arm over such a plurality of conductive pads a problem occurs at the end of each row or linear array of conductive pads.
The absence of additional conductive pads beyond the last pad in a linear array often causes the solder which has been deposited onto the last few conductive pads in a linear array to be "dragged" off the last few conductive pads by the solder nozzle, leading to a condition known as "solder starvation." This condition is generally thought to be brought about by the lack of additional conductive pads beyond the last few in an array and/or by the abrupt change of direction of the solder nozzle which must necessarily occur to permit the next linear array of conductive pads to be coated with a desired thickness of solder.
Thus, it should be apparent that a need exists for a method and apparatus wherein solder may be deposited in a uniform manner and thickness on a plurality of conductive pads which are disposed in one or more linear arrays on the surface of a circuit board.